1. Field of the Invention
The invention relates to an image forming apparatus using an electrophotographic system, and a developing device and a process cartridge for use in the image forming apparatus.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2003-263012 discloses an image forming apparatus such as a copier and a printer in which at least two carrying members among carrying members forming a developer circulation route in the longitudinal direction are arranged vertically in a developing device. The developer is a two-component developer consisting of a toner and a carrier that may contain additives.
A toner supplying port provided in the developing device using the two-component developer appropriately supplies the toner into the developing device depending on toner consumption in the developing device. The carrying member (a stirring and carrying member) such as a carrying screw stirs and mixes the supplied toner and the developer in the developing device. A part of the stirred and mixed developer is supplied to a developing roller (a developer carrier). A doctor blade (a developer regulating member) regulates the developer carried by the developing roller to an appropriate amount, and then the toner in the two-component developer attaches to a latent image on a photosensitive drum at a position facing the photosensitive drum. A magnet is fixed inside the developing roller, and the magnet forms a plurality of magnetic poles on the circumference of the developing roller.
A first carrying member (a first stirring and carrying member) and a second carrying member (a second stirring and carrying member) are arranged vertically in the developing device in Japanese Patent Application Laid-open No. 2003-263012. These two carrying members form the developer circulation route. The first carrying member arranged at an upper section of the developing device supplies the developer to the developing roller at a position where a developer attracting magnetic pole is located while carrying the developer in the longitudinal direction. The second carrying member arranged at a lower section of the developing device carries the developer released from the developing roller at a position where a developer releasing magnetic pole is located in the longitudinal direction (a direction opposite from the carrying direction of the first carrying member). The downstream side of the developer carrying route of the first carrying member (a first carrying route) connects to the upstream side of the developer carrying route of the second carrying member (a second carrying route) with a first relay unit (a relay unit). The developer reaching to the downstream side of the first carrying route drops on the first relay unit by its own weight and reaches to the upstream side of the second carrying route. The toner supplying port provided in the upstream side of the second carrying route appropriately supplies new toner. The upstream side of the first carrying route connects to the downstream side of the second carrying route with a second relay unit. The developer reaching the downstream side of the second carrying route, which is a mixture of the developer released from the developing roller, the developer dropped from the first relay unit, and the new toner supplied from the upstream side of the second carrying route, stays at the position, is pushed up, and moves to the upstream side of the first carrying route through the second relay unit.
Such a developing device in which the carrying members are vertically arranged is compacter in the horizontal direction compared with a developing device in which the carrying members are horizontally arranged. A developing device in which the carrying members are horizontally arranged is disclosed, for example, in FIG. 8 in Japanese Patent Application Laid-open No. 2003-263012. Therefore, a tandem type image forming apparatus in which a plurality of developing devices is horizontally arranged often uses the developing device in which the carrying members are vertically arranged. A developing device in which the carrying members are vertically arranged and that separates the developer supplying route to the developer carrier (the first carrying route) and the recovering route of the developer being released from the developer carrier (the second carrying route) can make a concentration deviation in the toner image formed on an image carrier because the developer carried on the developing roller and supplied to the developing process does not frequently include developer from after the developing process compared with the developing device in which the carrying members are horizontally arranged.
In the conventional technology disclosed in Japanese Patent Application Laid-open No. 2003-263012 and the like, the developer carried on the developing roller after the developing process is not always released to the second carrying route. Therefore, the developer carried on the developing roller and supplied to the developing process includes the developer from after the developing process, and generates a concentration deviation in the toner image formed on the image carrier.
This has been a non-negligible problem especially when fabricating a smaller-diameter developing roller to downsize the developing device. In a smaller-diameter developing roller, the magnetic poles formed on the developing roller influence each other, and therefore the magnetic flux density of the developer releasing magnetic pole becomes large due to two magnetic poles sandwiching the developer releasing magnetic pole. Therefore, the retention force acting on the developer becomes large at the position of the developer releasing magnetic pole, and the developer carried on the developing roller after the developing process is not released sufficiently.